Mn ion concentration dependence of the photoacoustic and photoluminescence spectra of ZnS:Mn nanocrystals (abstract)

Abstract

ZnS nanocrystals doped with Mn2+ ions (ZnS:Mn) have attracted much attention in the past few years due to a change in optical properties in accordance with quantum confinement effects. ZnS:Mn nanocrystals exhibit an orange luminescence with a high quantum efficiency (∼18%) under the interband excitation of the host crystals. This spectacular result suggests that the ZnS:Mn nanocrystal system forms a new class of luminescent materials with many applications. To verify the mechanism of high quantum efficiency in ZnS:Mn nanocrystals, it is important to establish the optical absorption characters as an initial step. Few studies have been conducted on light absorption of ZnS:Mn nanocrystals directly because of the difficulty in using of conventional methods caused by strong scattering in the finely powdered structures. The optical absorption spectra of them can be obtained by applying the photoacoustic (PA) spectroscopy, which is a powerful technique for detecting small amounts of strongly scattered materials. We will show the PA difference spectra between ZnS:Mn and pure ZnS nanocrystals with different Mn ion concentrations. The peak position corresponds to the transition energy between A16 state (ground state) and E4 state, and it increases with the decrease of Mn2+ ion concentration, indicating a possibility of hybridization of the s–p states of the ZnS host and the d state of the Mn2+ ion. The intensity of optical absorption between those states increases linearly with the increase of Mn ion concentration, indicating that the optical absorption of Mn ion impurities is proportional to the concentration. We have obtained photoluminescence (PL) spectra with different Mn ion concentration. The PL peak of the Mn2+ transition in ZnS nanocrystals (the transition from T14 to A16 states) is 2.08 eV with a half width of 0.24 eV, which is slightly shifted to lower energy region when compared to the peak in the bulk sample, and is independent of the Mn ion concentration. The peak intensity of the PL spectrum shows a maximum, and decreases with higher Mn ion concentration region.